(1) Field of the Invention
This invention relates to an electromagnetic wave reflection-preventing material and electromagnetic wave reflection-preventing method which make it possible to prevent interference due to electromagnetic wave and to be of a thin-gage film and weight-saved.
(2) Description of the Prior Art
For the purpose of avoiding a malfunction of an electronic equipment by an electromagnetic wave, there have been proposed in the art a process which comprises coating an electrically-conduction coating composition onto a housing of the electronic equipment, an process which comprises forming a thin film of a metal such as zinc, aluminium, iron and copper by plating, cladding, deposition, etc. onto a plastic substrate, and so forth. However, the above process coating the electrically-conductive coating composition onto the housing has such disadvantages that an electromagnetic wave shielding effect is poor and that the effect may be reduced with time. On the other hand, according to the above process forming the thin metal film onto the plastic substrate, the electromagnetic wave is reflected in such a large amounts that a secondary electromagnetic wave interference problem may be raised.
Japanese Patent Application Laid-open No. 241098/90 discloses an electromagnetic wave-shielding film formed by drawing a geometrical pattern onto a film by use of an electrically-conductive metal, and shows that the electromagnetic wave-shielding film has good shielding properties. The above films proposed in the art may effectively function as a shielding material for preventing a leakage of the electromagnetic wave generated from the electronic equipment, or preventing a malfunction of the electronic equipment due to the electromagnetic wave from outside, but may not effectively function for preventing an electromagnetic wave interference such as a false image of a radar due to a reflection of the electromagnetic wave, for example, by a bridge, a building, etc., or the like.
For the purpose of preventing the above interferences due to the reflection of the electromagnetic wave, there are known electromagnetic wave absorbent materials which are prepared by dispersing ferrite or a mixture of ferrite with metal powder or carbon powder into an organic polymer.
However, for the purpose of achieving a practically available absorbing characteristics by use of the above materials, it is necessary for the above materials to have a weight of 4 kg/m.sup.2 or more and a film thickness of 1 mm or more even in the case of an electromagnetic wave having a narrow band frequency with an effective band width of 0.5 to 1 GHz exclusive, or to have a weight of about 12 kg/m.sup.2 or more and a film thickness of 4.5 mm or more in the case of an electromagnetic wave having a broad band frequency with an effective band width of 1 to 5 GHz. Therefore, in use, the above electromagnetic wave absorbent material has such disadvantages that it is so heavy in weight and so thick in film thickness as to show poor application and working properties, that in the case where it is applied to building structures, etc., it is necessary to take strength and balance of the building structure as a a whole into consideration, and so forth. Thus, it has been of a great demand to develop an electromagnetic wave reflection-preventing material which is of a thin-gage film and weight-saved so as to show good application and working properties, and has improved electromagnetic wave shielding power and electromagnetic wave reflection-preventing power.